1. Field of the Invention
The present invention relates to surface acoustic wave devices, and more particularly, to a surface acoustic wave device having a wafer level package (WLP) structure.
2. Description of the Related Art
Surface acoustic wave devices are used as band pass filters in wireless sections, such as radio-frequency (RF) stages and intermediate-frequency (IF) stages, of mobile communication terminals. In recent years, wireless sections in personal digital assistants (PDAs), such as cellular phones and smartphones, have been modularized, and accordingly, there has been a need for a reduction in the size and height of surface acoustic wave devices. Thus, packaging technologies for surface acoustic wave devices have been improved, and a WLP technology has been proposed in which a chip of a surface acoustic wave device is directly used as a package. In a surface acoustic wave device, interdigital transducer (IDT) electrodes are provided on a piezoelectric substrate, and the surface acoustic wave device is packaged so that a hollow space is provided above the IDT electrodes. In a WLP-type surface acoustic wave device, a piezoelectric substrate is directly used as a package that includes a hollow space.
In modularization of wireless sections in PDAs, such as cellular phones and smartphones, there is a need to modularize such wireless sections by sealing them with a resin in order to protect them against external stress and moisture. When an entire module is sealed with a resin (e.g., transfer molding), high pressure is applied to individual components that are sealed with the resin, and accordingly, there is a need for a method for allowing a surface acoustic wave device having a hollow space inside thereof to be capable of withstanding such high pressure and to be capable of withstanding a mold resin that tries to enter the surface acoustic wave device (i.e., a method for allowing the surface acoustic wave device to have mold resistance or pressure resistance and to have a lamination property or a sealing property).
In the related art, as a technology that ensures the mold resistance and the lamination property of a surface acoustic wave device, a technology for providing a partition-support layer (inner support layer), which functions as a spacer, in a hollow space has been proposed (see, for example, Japanese Patent No. 5141852). In Japanese Patent No. 5141852, the mold resistance of a surface acoustic wave device is ensured by disposing a partition-support layer, which is made of a resin, at a center portion of a hollow space so as to be vertically arranged between a piezoelectric substrate and a cover layer that define the hollow space.
Here, in order to further improve the mold resistance, providing a partition-support layer, which extends in the lateral and longitudinal directions when the piezoelectric substrate is viewed in plan, in the hollow space may be considered. FIG. 7 is a layout view of a hollow space of a surface acoustic wave device according to a reference example (a diagram when a piezoelectric substrate is viewed in plan). FIG. 7 illustrates a piezoelectric substrate 11, on which IDT electrodes (not illustrated) have been provided, outer-periphery-frame support layers 20a to 20d which cover the periphery of the piezoelectric substrate 11, partition-support layers 22a to 22c, which are disposed so as to partition the hollow space, and columnar electrodes 17a to 17h. Each of the partition-support layers 22a to 22c is vertically arranged on the piezoelectric substrate 11 in a region in which the IDT electrodes are not disposed, and the partition-support layers 22a to 22c support, together with the outer-periphery-frame support layers 20a to 20d, a cover layer (not illustrated) that defines a ceiling of the hollow space. When the piezoelectric substrate 11 is viewed in plan, the partition-support layer 22c has a substantially crank shape extending so as to partition the hollow space in the longitudinal and lateral directions. As a result, reinforcement strength with respect to the cover layer may be enhanced, and the mold resistance of the surface acoustic wave device may be improved, whereas if all the partition-support layers 22a to 22c extend in only one direction, the reinforcement strength with respect to the cover layer would not be enhanced, and the mold resistance of the surface acoustic wave device would not be improved.
However, in the case where the partition-support layer 22c having a substantially crank shape, such as that illustrated in FIG. 7 is provided, when the cover layer, which is made of a resin, is attached to the outer-periphery-frame support layers 20a to 20d and the partition-support layers 22a to 22c from above, wrinkles are generated in a portion of the cover layer that is brought into contact with a crank portion of the partition-support layer 22c, and the partition-support layer 22c is embedded into the cover layer. As a result, the portion of the cover layer in contact with the crank portion of the partition-support layer 22c, is crushed. For example, when attaching a cover layer, which has been rolled, to the partition-support layer 22a, the partition-support layer 22b, and the partition-support layer 22c in this order while stretching the cover layer, when the cover layer is attached to the partition-support layer 22c, wrinkles are generated in the cover layer, and the cover layer is crushed. As a result, problems occur in that a portion having a reduced thickness is locally provided in the cover layer, or breakage occurs locally in the cover layer, so that the mold resistance and the lamination property of the surface acoustic wave device will not be ensured, and the reliability of the surface acoustic wave device will not be ensured as a result of the wrinkles, which have been generated in the cover layer, being in contact with the IDT electrodes or wiring electrodes.